1. Field of the Invention
The present invention is generally related to a system for inhibiting the corrosion of components within a marine propulsion system and, more particularly, to a system with primary and secondary corrosion inhibiting devices that are configured to work cooperatively with each other while avoiding the disadvantageous results that can sometimes occur when two cathodic systems are both used on a common marine vessel.
2. Description of the Related Art
Those who are skilled in the art of marine propulsion systems are familiar with various techniques that can be used to inhibit the corrosion of submerged components through the formation of galvanic circuits. Those skilled artisans are also familiar with various techniques used to avoid the formation of those galvanic circuits that can otherwise degrade and erode the surface of metallic components used in marine propulsion systems.
U.S. Pat. No. 2,571,062, which issued to Robinson et al. on Oct. 9, 1951, describes a sacrificial anode system for protecting metals in seawater. The tendency for structures of steel and similar metals, when immersed in seawater, to undergo serious corrosion can be offset by cathodic protection. In this process the structure is made the cathode in an electric circuit using the seawater as an electrolyte. If sufficient current is supplied, the structure can be kept from corroding.
U.S. Pat. No. 3,242,064, which issued to Byrne on Mar. 22, 1966, describes a cathodic protection system. It relates to corrosion reduction systems in which the direct current supplied to the surface to be protected, such as the hull of a ship, is automatically varied in accordance with the protective conditions on the hull, as monitored by a sensing half-cell.
U.S. Pat. No. 3,327,214, which issued to Allen et al. on Jun. 20, 1967, describes an electronic current meter having linear response. It relates to an electronic meter and, more particularly, to one used in procedures for determining the current requirements for cathodic protection of well casings and the like. The current required for the cathodic protection of well casings and the like can be determined by the polarization curve method. In this method, cathodic currents are applied to the well casing in discreet increments. At each current increment, the current is momentarily interrupted and the casing-to-soil polarization potential, with respect to a reference electrode placed in the earth some distance from the well head, is determined. The difference between these measured polarization potentials with each increase in current are normally of the order of a few millivolts.
U.S. Pat. No. 3,953,742, which issued to Anderson et al. on Apr. 27, 1976, discloses a cathodic protection monitoring apparatus for a marine propulsion device. The monitor is coupled to an impressed current cathodic protection circuit used for corrosion protection of a submerged marine drive. The cathodic protection circuit includes one or more anodes and a reference electrode mounted below the water line and connected to an automatic controller for supplying an anode current which is regulated in order to maintain a predetermined reference potential on the protected structure. A switch selectively connects a light emitting diode (LED) lamp or other light source between the controller output and ground so that the controller current may, when tested, be used to operate the light source in order to confirm that power is available to the anode.
U.S. Pat. No. 4,322,633, which issued to Staerzl on Mar. 30, 1982, discloses a marine cathodic protection system. It maintains a submerged portion of a marine drive unit at a selected potential to reduce or eliminate corrosion thereto. An anode is energized to maintain the drive unit at a preselected constant potential in response to the sensed potential at a closely located reference electrode during normal operations. Excessive current to the anode is sensed to provide a maximum current limitation. An integrated circuit employs a highly regulated voltage source to establish precise control of the anode energization.
U.S. Pat. No. 4,445,989, which issued to Kumar et al. on May 1, 1984, describes a ceramic anode for corrosion protection. The anode is useful in corrosion protection comprising a metallic substrate having an applied layer thereon of a ferrite or a chromite, is described. The layer having metallic is electronic conductivity and a thickness of at least 10 mils is used.
U.S. Pat. No. 4,492,877, which issued to Staerzl on Jan. 8, 1985, discloses an electrode apparatus for cathodic protection. The apparatus is provided for mounting an anode and reference electrode of a cathodic protection system on an outboard drive unit. The apparatus includes an insulating housing on which the anode and reference electrode are mounted and a copper shield mounted between the anode and electrode to allow them to be mounted in close proximity to each other. The shield is electrically connected to the device to be protected and serves to match the electrical field potential at the reference electrodes to that of a point on the outboard drive unit and remote from the housing.
U.S. Pat. No. 4,528,460, which issued to Staerzl on Jul. 9, 1985, discloses a cathodic protection controller. A control system for cathodically protecting an outboard drive unit from corrosion includes an anode and a reference electrode mounted on the drive unit. Current supplied to the anode is controlled by a transistor, which in turn is controlled by an amplifier. The amplifier is biased to maintain a relatively constant potential on the drive unit when operated in either fresh or salt water.
U.S. Pat. No. 4,872,860 which issued to Meisenburg on Oct. 10, 1989, discloses a sacrificial anode for marine propulsion units. The anode is disposed in association with the trim cylinder unit of a marine propulsion device and is positioned in the previously unused area between the aft cylinder end and the rodeye or the like on the piston rod end. More specifically, the anode is in the form of an elongated generally cylindrical member of a diameter approximately that of the trim cylinder to provide improved mass characteristics, and is deeply grooved to thus provide ribs which enhance the working surface area. The anode may be attached to an extended pilot member which is suitably secured within the aft end of the trim cylinder.
U.S. Pat. No. 5,627,414, which issued to Brown et al. on May 6, 1997, describes an automatic marine cathodic protection system using galvanic anodes. The system provides a controlled and optimum amount of cathodic protection against galvanic corrosion on submerged metal parts. Intermittently pulsed control circuitry enables an electro-mechanical servo system to control a resistive element interposed between the sacrificial anodes and the electrically bonded underwater parts. In an active mode of operation a current is applied directly to the anodes to quickly establish the proper level of correction which is maintained during the passive mode.
U.S. Pat. No. 5,716,248, which issued to Nakamura on Feb. 10, 1998, describes a sacrificial anode for marine propulsion units. Various anode arrangements for marine propulsion units are described wherein the sacrificial anode is juxtaposed to the trim tab and is detachably connected to the lower unit housing by fastening means which can be removed from the upper surface thereof. In one embodiment, the trim tab is detachably connected to the sacrificial anode and connected to the outer housing portion through the sacrificial anode.
U.S. Pat. No. 5,747,892, which issued to Staerzl on May 5, 1998, discloses a galvanic isolator fault monitor. A system and method for testing and monitoring the operation of a galvanic isolator is disclosed. The galvanic isolator is positioned between shore ground and boat ground to prevent the flow of destructive galvanic currents between the shore ground and the boat ground. The monitoring system transmits a test current through the galvanic isolator at specific time internals to test the effectiveness of the galvanic isolator. The monitoring system includes a first counter that outputs an enabling signal after a desired period of time. The enabling signal allows a test current to flow through the galvanic isolator for a brief period of time determined by a second counter.
U.S. Pat. No. 5,840,164, which issued to Staerzl on Nov. 24, 1998, discloses a galvanic isolator. It is intended to protect against galvanic corrosion of a submersible metal marine drive. The galvanic isolator is positioned between shore ground and boat ground to prevent the flow of destructive galvanic currents between those grounds while maintaining the safety function of neutral ground. The galvanic isolator of the invention includes a blocking element positioned between the boat ground and the shore ground that can be switched between an opened and a closed state by a trigger circuit. The trigger circuit closes the blocking element when the voltage difference between the boat ground and the shore ground exceeds a threshold value, such as 1.4 volts. During operation of the galvanic isolator during the high fault current situation, power is dissipated only by the blocking element, rather than by the combination of the blocking element and the trigger device. In this manner, the galvanic isolator reduces the amount of power dissipated during high current conditions and therefore reduces the amount of heat generated by the galvanic isolator.
U.S. Pat. No. 6,183,625, which issued to Staerzl on Feb. 6, 2001, discloses a marine galvanic protection monitor. The system uses two annunciators, such like light emitting diodes, to alert a boat operator of the current status of the boat's galvanic protection system. A reference electrode is used to monitor the voltage potential at a location in the water and near the component to be protected. The voltage potential of the electrode is compared to upper and lower limits to determine if the actual sensed voltage potential is above the lower limit and below the upper limit. The two annunciator lights are used to inform the operator if the protection is proper or if the component to be protected is either being overprotected or underprotected.
U.S. Pat. No. 6,547,952, which issued to Staerzl on Apr. 15, 2003, discloses a system for inhibiting fouling of an underwater surface. An electrically conductive surface is combined with a protective surface of glass in order to provide an anode from which electrons can be transferred to seawater for the purpose of generating gaseous chlorine on the surface to be protected. Ambient temperature cure glass (ATC glass) provides a covalent bond on an electrically conductive surface, such as nickel-bearing paint. In this way, both hulls, submerged portions of outboard motors, and submerged portions of sterndrive systems can be protected effectively from the growth of marine organisms, such as barnacles.
U.S. Pat. No. 7,064,459, which issued to Staerzl on Jun. 20, 2006, discloses a method of inhibiting corrosion of a component of a marine vessel. A method for inhibiting galvanic corrosion of marine propulsion components impresses an electronic current into the protected component and causes the protected component to act as a cathode in a galvanic circuit which comprises a conductor, such as a ground wire connected between the protected component and an electrical conductor which is external to the marine vessel on which the protective component is attached. The electrical conductor can be a ground wire of an electrical power cable connected between the marine vessel and the shore ground. The sea bed is caused to act as an anode in the galvanic circuit, with varying voltage potentials existing within the water between the sea bed and the protected component. The system can be a closed loop control circuit using a voltage sensed by an electrode, or an open loop circuit that provides current pulses based on empirical data.
U.S. Pat. No. 7,381,312, which issued to Misorski et al. on Jun. 3, 2008, discloses a cathodic protection system for a marine propulsion device with a ceramic conductor. A ceramic conductor is supported by an electrically insulative support member for attachment directly to a marine propulsion drive and for use as either an anode or electrode in a corrosion prevention system. The ceramic conductor is received within a depression formed in a surface of the electrically insulative support member and the exposed surface of the ceramic conductor can be offset from or coplanar with an exposed surface of the electrically insulative support member. The ceramic conductor can comprise oxides of iridium, tantalum and titanium that are formed as a coating on a titanium substrate.
U.S. Pat. No. 7,387,556, which issued to Davis on Jun. 17, 2008, discloses an exhaust system for a marine propulsion device having a driveshaft extended vertically through a bottom portion of a boat hull. The exhaust system directs a flow of exhaust gas from an engine located within the marine vessel, and preferably within a bilge portion of the marine vessel, through a housing which is rotatable and supported below the marine vessel. The exhaust passageway extends through an interface between the stationary and rotatable portions of the marine propulsion device, through a cavity formed in the housing, and outwardly through hubs of pusher propellers to conduct the exhaust gas away from the propellers without causing a deleterious condition referred to as ventilation.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
Those skilled in the art of marine propulsion systems and corrosion inhibiting devices are familiar with the fact that two basic approaches have been used for many years to inhibit galvanic corrosion. One technique involves the use of a sacrificial anode which, as the name implies, uses an anode that is sacrificed in order to protect a more important or valuable device, such as an aluminum marine drive unit. The sacrifice involves the gradual corrosion and, potentially, the eventual disappearance of the material of which the sacrificial anode is made. This material typically comprises zinc, magnesium, or aluminum because of their relative potential difference to the material that they are protecting which can be summarized in a table called the galvanic series. A circuit using this technique typically selects a material with an electrode potential that is more negative than the material of the component being protected. As an example, using the values from Table III, if the goal is to protect an iron component (electrode potential of −700 mV) it would be possible to use an aluminum anode (electrode potential of −1075 mV) as the sacrificial component because the aluminum would sacrifice itself by giving up electrons to protect the iron. Another example could use a zinc anode (electrode potential of −1150 mV) in order to protect a copper device (electrode potential of −300 mV).
Another technique that can be used to inhibit galvanic corrosion is a system that impresses a current into the protected component in order to raise its potential and cause it to act as a cathode in the circuit which connects the protected component (i.e. the cathode) electrically with the sacrificial component (i.e. the anode) with a conductor (i.e. a wire or other current path between the protected component and the protecting component). This forms a half cell. Another half cell is made up of the sacrificial and protected components along with an electrolyte (i.e. water) in which they are both submerged. The conductor provides a path through which electrons can flow from the anode to the cathode as the ions move through the electrolyte from the cathode to the anode.
Since both of these techniques are available to the designer for the purpose of inhibiting galvanic corrosion, in many systems both techniques are used in the same design. This provides both primary and secondary corrosion inhibiting systems. However, as will be described in greater detail below, the presence of both systems can lead to disadvantageous interactions in which the efficiency of the total system is decreased. As will be described in greater detail below, it would be significantly beneficial if a system could be provided to allow the use of primary and secondary systems in a way which avoids the disadvantageous interactions between them. It would also be beneficial if the system could also be directed toward the goal of avoiding the counterproductive interference between primary and secondary systems in the ways that are prevalent in the prior art.